The present invention is concerned with molding, specifically by a technique employing freezing and pressure, of metallic powders.
Products molded from metallic powders are coming into more and more common use, thanks to the many advantages they offer over items machined from fused bodies, and compression molding using a molding die is being adopted as a means of mass producing them. The problem is, however, that only relatively simple shapes can be produced by this method. To make objects of complex configuration, and particularly those that are stepped in the direction in which pressure is applied such as boxes, the density of the compressed powder body varies from place to place so that it is impossible to obtain satisfactory products.
To overcome this, an injection molding method has been proposed. However because of the poor flowability of the powder feedstock itself, it is difficult to fill the mold cavity for complex shapes uniformly to the very corners, and because of friction between the powder and the die walls, it is not possible in practice to apply sufficient molding force.
To counteract this, a method has been used in which powdered resin is added to the powder feedstock as a binder, heat is applied to melt the resin, and the resulting flowability utilized in effect molding.
However the application of this method involves the use of considerable amounts of resin binder, so that the product is actually plastic, with a high metallic content.
This led to the following problems:
1. Because of the admixture of a large amount of resin binder, the sintered body tends to be porous. This, coupled with a large amount of shrinkage makes it difficult to guarantee the high dimensional accuracy and high density suitable for machine components.
2. If dewaxing (the removal of the resin binder by heating and decomposing into gas prior to sintering) is carried out too precipitously, scaling and deformation occur. To avoid this, the rate of temperature rise must be reduced and high temperatures cannot be used. As a result, the process of removing the binder requires an inordinate amount of time. As a result, productivity is reduced, and vast amounts of heat are required, leading to higher production costs.
3. When a resin binder is used, it is mixed with the metallic powder, heated, and injected into the mold. Since, however, the viscous resistance of the binder is greater, the behavior of the binder when flowing gives rise to uneven distribution of the powder in the molded object, which tends to manifest itself after sintering as product defects. In places where the resin binder flows readily, the powder density is lessened, while it becomes correspondingly greater in the corners. Furthermore, the resin may be concentrated along the weld line (the flow front of the mixture) so that a resin binder layer is present on the surface leading to increased surface roughness after sintering.
4. If the amount of resin binder is reduced, molding parameters such as the pressure and temperature of injection become more critical and harder to control.